The present invention relates to smoke detectors and in particular, relates to a method of calibrating a smoke detector. The invention also relates to a smoke detecting system where the alarm panel communicates with a series of calibrated smoke detectors.
Many smoke detectors include a light emitting diode (LED) light source that produces a light beam within a smoke detecting chamber. A photo diode is positioned to receive light that is scattered by smoke particles in the smoke chamber. The walls of the smoke chamber have a series of passages for allowing smoke particles to flow into or out of the chamber. The walls of the chamber are also designed to reduce the amount of light reflected by the walls back into the chamber. A processing circuit is associated with the photo detector to measure the amount of light received.
The various components of the smoke detector all collectively contribute to the sensitivity of the detector and the detector at the time of manufacture requires calibration. One of the main factors that lead to vary significant tolerance variations is the output of the LED light source. The output of the LED is adjusted to vary the sensitivity of the smoke detector. The calibration of smoke detectors to date has involved the adjustment of the output of the LED to achieve a particular alarm threshold measured by the photo detector for a known level of obscuration. Unfortunately, due to the significant variations in the tolerance of the LED, a considerable variation in the sensitivity of the smoke detector at various obscuration points occurs when this method of calibration is used.
To overcome this problem, it is possible to use LEDs with a smaller tolerance range; however, the problem is only reduced and the cost increases substantially.
The calibration method of the present invention reduces the problems associated with tolerance variation impact on calibration.